Acoustic device



1941- i G; M. GIANNINI EI'AL I 2,252,846

' ACOUS TIC DEVICE Filed Sept. 50, 1958 4 Sheets- Sheet 1 9 M4324 Z M5,!P5 I C 7 (M2) FIGLI INVENTORS GABRIEL M. GIANNINI M ICHARD. w. CARLI S; ATTORNE.

Aug. 19, 1941. QMGMNNINWM 1,252 8 6 ACOUSTIC DEVICE Filed Sept. so, 1938 4 Sheets-Sheet 2 M/ M2 R/ C;/

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FIG. 8

INVENTORS: GABRIEL M.G|ANN|NI BY I RICHARDWCARLILE ATTORNEY.

Aug. 19,1941. G. M. GVIANVNINI EI'AL ACOUSTIC DEVICE 4 SheetsShee t 3 Filed Sept. 30, 1938 IN VENTORS IOWI) Aug. 19, 1941; G. M. GIANNINI EI'AL ACOUSTIC. DEVICE Filed Sept. 50, 1938 4'Sheets-Sheet 4 INVENT GABRIEL M.G|AN l. 10%! CARLISL ATTORNEY.

Patented Aug. 19, 1941 ACOUSTIC DEVICE Gabriel M. Giannini, Great Neck, and Richard W. Carlisle, Greenbur'g, N. Y., assignors, by mesne assignments, to Associated Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application September 30, 1938, Serial No. 232,658

Claims. This invention relates to acoustic devices, and,

more particularly, to electrodynamic translating devices of the type used as both microphone and loudspeaker in connection with switching devices whereby it is used alternately as microphone and loudspeaker.

An object of the invention is to improve the frequency response of a dynamic microphone, and to make a more sensitive dynamic microphone than has been made heretofore.

Another object is to construct a microphone of this class which is completely sealed against dust and moisture.

Another object is to improve the radiating qualities of a small electrodynamic device so as to make it useful as a loudspeaker without a baffle 0r cabinet. 1

Another object is to provide a shock-proof ascoercive force magnets'which are somewhat brittle.

A preferred embodiment of this invention comprises a transducer having a housing containing a voice coil and diaphragm assembly, a resilient mounting therefor, a magnetic structure having a longitudinally disposed core so arranged as to cause a constant magnetic flux to emanate from said core and permeate said voice coil, and various acoustic enclosures coupled to said diaphragm. In general, these consist of a small chamber behind and adjacent to the diaphragm, a pair of chambers within the magnetic structure with vents to the first-mentioned chamber, and a fourth chamber within the casing behind the magnetic structure, this being vented to each chamber within the magnetic structure.

The primary factors which combine to give good sensitivity as a microphone and good lowfrequency radiation as a loudspeaker are the large number of acoustic resonances induced throughout the frequency range by the various acoustic chambers, the great flexibility with which the diaphragm and voice coil assembly are suspended and the specific acoustic labyrinth by which the effective mass of the diaphragm is augmented at low frequencies by air flowing past the voice coil into the inner air chamber surrounding the voice coil within the magnetic structure.

The specific manner in which this is made to occur will be explained in connection with the drawings, in which Fig. l is a sectional view of an acoustical device constituting a preferred embodiment of this invention.

Fig. 2 is a diagram of an electrical circuit in which the various elements and their arrangements are mathematically analogous to the microphone of Fig. l.

Fig. 3 is a schematic sectional view of the same microphone as Fig. 1 showing the principal acoustic flow paths which occur at very low frequencies.

Fig. 4 is a. simplification of Fig. '2in which all elements not important at low frequencies have been omitted.

Fig. 5 is a schematic sectional view similar to Fig. 3 but showing the principal acoustic flow paths which occur at a high frequency.

Fig. 6 is a simplification of Fig. 2 in which all elements not important at high frequency have been omitted.

, sembly in order to take advantage of new high Fig. 7 is another simplification of Fig. 2 in which all elements not specifically important at very high frequencies have been omitted.

Fig. 8 is an alternative diaphragm construction.

The transducer shown in Fig. 1 is in general comprised of a housing I, a snap-on front rim 2 for said housing, a front screen 3, a resilient bushing member 4, and an inner unit designated as assembly 5. This assembly is comprised of the magnetic assembly 6 and additional parts including the diaphragm 1 and clamping rings 8 and 9. The voice coil I0 is aflixed to the diaphragm I as indicated, and is mainly disposed in the air gap formed between the pole core II and the flat plate [2. The magnet I3 is of annular shape and is clamped between the pole plate l2 and. the opposing pole plate It in any convenient manner, many of which are well known in the art.

The chamber bounded on the outside by the magnet I3 is divided into two annular chambers C5 and C6 by the cylindrical partition member is which is sealed at its ends to the pole plates l2 and H. The outer chamber C5 is vented to the chamber C3 adjoining the diaphragm I by a r plurality of apertures in plate l2, the aggregate inertance and resistance of which are designated by M5 and R5, respectively. The outer chamber C6 is also vented through the plate It by orifices having an aggregate inertanceand. resistance M6, Rs, respectively. The inner chamber C5 is vented through the plate It by orifices having an aggregate inertance and resistance of M1, R7, respectively.

The compliance in mechanical units of the various chambers is C3, C5, C6 and C1, the figures Fig. l.

The diaphragm assembly is comprised of the I diaphragm I having a generally conodial shape, rounded in the middle to avoid the necessity for deeply undercutting the pole core Ii. The diaphragm I is aflixed around its outer periphery to the flat ring ii, of varnished silk, which is rigidly aflixed at its outer periphery between the clamping rings 8 and 9. Two threads I! are passed through close-fitting holes in the diaphragm I and fastened in holes in ring 9, said threads being cemented into the diaphragm where they pass through it. In this manner a positive centering control for the voice coil i is assured without the introduction of appreciable stiffness into the system.

The performance of this device may be explained to persons skilled in the art of electric circuit analysis by reference to the circuit mathematically equivalent to the acoustic-mechanical structure.

An efiiciency has been attained in this device far in excess of any previously-developed device of this class, by dividing the audible frequency band into a number of bands and making speciflc provision in the structure for lowering the acoustic and mechanical impedance in each band until the desired performance was attained.

The complete equivalent electrical circuit is shown in Fig. 2. The electrical elements are labelled withthe same letters and subscripts-as their mechanical or acoustic equivalents. The elements not heretofore described are as follows: CzRz represent the compliance and resistance within the structure of the diaphragm I in the annular region thereof juxtaposed to the voice coil i0. C4 is the compliance (very small) of the chamber bounded by the middle of the diaphragm I and the end of the pole core I i. T1-'I'2 represent an electrical transformer (included.

only for the purpose of maintaining a strict analogy) which is intended to take intoaccount the fact that the volume of air flow through the inner side of the voice coil in (a slit having inertance M: and resistance R3) is much less than that through the outer side thereof because the diaphragm area within the voice coil is much less than that outside of said voice coil. The

turn ratio TizTz is therefore in the proportion of inner area and outer area.

Since the complete equivalent circuit of Fig. 2 is very complicated, simplified circuits have been worked out in combination with charts of the acoustic system, simplified with a view towards showing the manner in which various acoustic resonances tend to accentuate the response of the device. Figs. 3 and 4 may be considered together for the purpose of explaining the action of the device at extremely low frequencies, say 60 cycles per second. Acoustic waves A impinging upon the diaphragm 1 cause motion of said diaphragm, compressing the air in chamber C: and forcing some air through the slit around the voice coil I! which has inertance M4 and resistance R4. This air then passes into chamber C5, then through the orifices M1 R1 into chamber C1. The slit M4 has very small cross-section and consequently the inertance M4 is high. This inertance M4 adds effectively to the masses M1 and M2 of the voice coil and diaphragm respectively, and since inertance M1 is made low, the effective compliance of chamber Cu is directly additive to that of chamber C1, which is large. Due to the high efiective m s referring to the various chambers as indicated in of the system and the lowered compliance, a low natural resonance frequency results, with high sensitivity of action, and with no connection between the rear of the diaphragm and the outer atmosphere. The equivalent circuit of Fig. 4 may be used for calculations by inserting suitable numerical values for the various elements. It may be mentioned in connection with the discussion of air flow that relatively little air passes through the vents MsRs at low frequencies because they are padded with a predetermined amount of acoustic resistance material such as cotton, wool or silk yarn or felt.

In a similar manner the operation of the system at a fairly high frequency, say 3,000 cycles, may be explained by reference to Figs. 5 and 6. Sound waves A impinging upon the diaphragm 1 cause compression of air in chamber C3. Due to the high inertance of the air around the voice coil ID, no air can pulsate past it at high frequencies and it chooses the lower inertance path through the vents MsRs into chamber Cs, then through the vents McRs into chamber C1. Since each vent is padded the acoustic resistance is high and the entire acoustic system constitutes an acoustic damping system, by which a certain degree of resonance is built, up between the voice coil and diaphragm masses and the compliance of chamber C3; then this resonance is nicely damped by the coupling to C3 of the remaining acoustic orifices and chambers. This results in a system which is extremely sensitive within the frequency range considered.

Thissensitivity is due not merely to the two resonances specifically described in connection with Figs. 3 and 5, but to a number of other resonances occurring in the frequency range between there two resonances. For instance, the transition between the frequencies at which air can and cannot pass through the voice coil slit is gradual, and Various resonances occur between the masses of the voice coil, diaphragm and the slit inertance on one hand and the compliances oi chambers C3, C5, C5 and C1 on the other hand.

The mass of the diaphragm I varies with frequency, becoming progressively less as the frequency is raised. This is a well-known characteristic of conoidal diaphragms. It is taken advantage of in this device to introduce an additional resistance at the upper end of the frequency spectrum utilized, say around 4000 cycles. This is indicated in the equivalent of Fig. '7, C2 and R2 having been hereinbefore described as the compliance and resistance properties of the diaphragm in the annular region adjacent to the voice coil Ill. The resonance between M1, C2 and M2 is valuable in maintaining the response at a high level for an additional frequency band.

When the device is used as a loudspeaker, the previous discussion remains valid except that instead of response it has "radiation of substantially the same proportional degree of efflciency.

When a given acoustic translating device is used as both microphone and loudspeaker, it will not necessarily have the same frequency response in each application.

-tive at low frequencies; while the same .device used as a loudspeaker would sound progressively In the case of this particular device, flat over-all response of the' louder as the frequency is increased, up to its cutoff frequency. v V

In' an alternative construction, the diaphragm I may have its central portion i8 cut out as shown in Fig. 8. This makes it unnecessary to machine out the end of the pole core II to clear the tip of the diaphragm as was indicated in Fig. 1.

Other modifications. of this device will occur to those skilled in the art.

What is claimed is:

1. In an electro-acoustic translating device, in combination, a voice coil and diaphragm assembly, resilient mounting means therefor, a magnetic structure having a longitudinally disposed core so arranged as to cause a constant magnetic flux to emanate from said core and permeate said voice coil, means for enclosing the side of said diaphragm disposed toward said magnetic structure, means for dividing the air volume within said magnetic structure into a plurality of chambers one of which surrounds and extends the full length of said core and the other of which surrounds the full length of said first chamber, an additional acoustic enclosure abutting on said magnetic structure, and a plurality of acoustic vents in one end of said magnetic structure connecting each of said plurality of chambers with said additional inclosure, whereby a plurality of acoustic inertances and compliances are constituted and made to reenforce the action of said diaphragm over a wide frequency band. i

2. In an electro-acoustic translating device, in combination, a voice coil and diaphragm assembly, resilient mounting means therefor, a magnetic structure having a longitudinally disposed core so arranged as to cause a constant magnetic flux to emanate from said core and permeate,

said voice coil, means for enclosing the side of said diaphragm disposed towards said magnetic structure, means for dividing the air volume within said magnetic structure into a plurality of chambers one of which surrounds and extends the full length of said core and the other of which surrounds the full length of said first magnetic structure into a plurality of chambers one of which surrounds and extends the full length of said core and the other of which surrounds the full length of the chamber surrounding the core, and a plurality of acoustic vents in one end of said magnetic structure connecting said first chamber and one of said plurality of chambers within said magnetic structure, and an air gap for said voice coil constituting an acoustic vent between said first chamber and another of said plurality of chambers within said magnetic structure, whereby a plurality of acoustic inertances and compliances are constituted and made to reenforce the action of said diaphragm over a wide frequency band.

4. In an electro-acoustic translating device, in combination, a voice coil and diaphragm assembly, resilient mounting means therefor, a magnetic structure having a longitudinally disposed core so arranged as to cause a constant magnetic flux to emanate from said core and permeate said voice coil, means for enclosing the side of said diaphragm disposed towards said magnetic StIliCr ture to form a first acoustic chamber, an annular partition member surrounding the full length of said core and disposed in spaced relationship therefrom for dividing the air volume within said magnetic structure into a plurality of chambers one of which surrounds and extends the full length of said core and the other of which surrounds the full length of the chamber surrounding the core, said partition member being eccentrically disposed with respect to said core, and a plurality of acousticvents in one end of said magnetic structure connecting said first chamber and one of said plurality of chambers within said magnetic structure, and an air gap for said voice coil constituting an acoustical vent between said first chamber and the chamber surrounding said core, whereby a plurality of acoustic inertances ,and compliances are constituted-and made to chamber, a housing, an additional acoustic enclosure abutting on said magnetic structure and bounded by said magnetic structure and said housing, and a plurality of acoustic vents in one of said plurality of chambers with said additional inclosure, whereby a. plurality of acoustic inertances and compliances are constituted and made to reenforce the action of said diaphragm over a wide frequency band.

end of said magnetic structure connecting each 3. In an electro-acoustic translating device, in I combination, a voice coil and diaphragm assembly, resilient mounting means therefor, a magnetic structure having a longitudinally disposed core so arranged as to cause a constant magnetic flux to emanate from said core and permeate said voice coil, means for enclosing the side of said diaphragm disposed towards said magnetic structure to form a first acoustic chamber, an annular partition member surrounding the full length of said core and disposed in spaced relationship therefrom for dividing the air volume within said reenforcethe action of said diaphragm over a wide frequency band.

5. In a sound translating device, a voice coil and diaphragm assembly, a magnetic structure therefor, means for enclosing said magnetic structure and the side of said diaphragm assembly including said voice coil, a first acoustic chamber formed thereby between the pole face of said magnetic structure and said diaphragm, a second acoustic chamber formed thereby between the rear of said magnetic structure and said enclosing means, a third and fourth acoustic chamber within said magnetic structure, a plurality of'acoustic vents in both ends of said magnetic structure acoustically connecting said third chamber with said first and second chambers to enhance the action of said diaphragm at high frequencies, an air gap for said voice coil in the pole face of said magnetic structure, a plurality of acoustic vents in the rear of said magnetic structure, said air gap and said vents acoustically connecting said fourth chamber with said first and second chambers-to enhance the action of said diaphragm at low frequencies.

GABRIEL M. GIANNINI. RICHARD W. CARLISLE. 

